Conventional digital signal processing is achieved by sampling a signal, converting said samples into digital values and then performing mathematical operations over said digital values to achieve the desired processing.
In many applications, one has to switch from one sampling rate to another in the course of the process. Assuming the rates are multiples to one another, up sampling may be achieved by interpolating in between consecutive samples, while down sampling would simply require regularly dropping samples. This process does not directly apply to systems wherein sampling rates ratio would not be an integer value or in other words sampling rates relatively prime.
A fairly straightforward solution would be achieved by converting the sampled signal back to analog form and resampling it at proper rate. However, this solution would not only lack efficiency from a processing load standpoint, it might add to the coding error.
An improved method is disclosed in U.S. Pat. No. 4,460,890 wherein direct digital to digital conversion from a first sampling rate to a second sampling rate, not multiple to each other, is achieved using a rate multiplication means feeding an interpolating means.
This solution is still considered rather complex and not cost optimized.